Presently, two Gi/o protein coupled cannabinoid receptors have been characterized in mammals and other organisms: CB1, a receptor found in the mammalian brain and a number of other sites in peripheral tissues; and CB2, a peripheral receptor found principally in cells related to the immune system. Compounds known as cannabinergic ligands bind to, and thereby modulate, CB1 and/or CB2 receptors in a subject. Such modulation of CB1 and/or CB2 receptors can produce numerous physiological effects in a subject. In vitro methods for assaying the ability of a compound to bind to CB1 and/or CB2 receptors are known, and results from these assays correlate with, and predict, the in vivo ability of that compound to bind to CB1 and/or CB2 receptors.
Despite having a rapid onset of action, the magnitude and duration of in vivo CB1 and/or CB2 receptor modulation by cannabinergic ligands are relatively short, because of a rapid inactivation process comprising hydrolysis of that cannabinergic ligand. Fatty acid amide hydrolase (FAAH) and monoacylglycerol lipase (MGL) are two enzymes responsible for the termination of endocannabinoid signaling. FAAH (also referred to as anandamide amidase (AEAase) and anandamide amidohydrolase (AAH)), is an intracellular membrane-bound enzyme that degrades and inactivates members of the endocannabinoid class of signaling lipids such as anandamide(arachidonoyl ethanolamine). FAAH belongs to the amidase signature (AS) super family of serine hydrolases and, in contrast to the classical serine-histidine-aspartate triad found in most serine hydrolases, the catalytic machinery of this enzyme is a serine-serine-lysine catalytic triad. FAAH has been isolated, molecularly cloned and its 2.8 Å crystal structure was recently reported.
FAAH can not only hydrolyze anandamide into arachidonic acid and ethanolamine but it can also catalyze its reverse synthesis from the two hydrolysis components. FAAH can also hydrolyze several bioactive fatty acid amides not belonging to the endocannabinoid family, including, but not limited to, the sleep inducing lipid oleamide, the appetite-suppressing agent oleoylethanolamine, the related 1-oleoylglycerol and 2-oleoylglycerol, and the peripheral analgesic and anti-inflammatory mediator palmitoylethanolamine. In general, FAAH can hydrolyze a range of fatty acid amides, ethanolamides, esters, arachidonoyl substrates, and oleoyl substrates, as well as other substrates.
Although FAAH has been shown to catalyze hydrolysis of 2-arachidonoylglycerol (another endogenous cannabinergic ligand) in vitro, MGL plays the predominant role in catalyzing 2-AG hydrolysis in vivo. MGL (also known as MGL, MAG lipase or MAGL) is a serine hydrolase that converts 2- and 1-monoglycerides to fatty acid and glycerol.
The physiological responses that result from modulation of cannabinoid receptors have recognized therapeutic potential. (see Saario and Laitinen (2007) Basic & Clin. Pharmacol. Toxicol. 101:287-292; Jhaveri et al. (2007) Brit. J. Pharmacol. 152:624-632; Karanian et al. (2007) J. Pharmacol. Expt. Thera. 322:1059-1066; Scherma et al. (2008) Neuorophmarmacol. 54:129-140. However, the effectiveness of these responses is limited and short-lived due to the hydrolytic activity of FAAH and MGL.
A need, therefore, exists for compounds that inhibit the hydrolytic activity of FAAH and/or MGL, thereby maintaining or increasing the magnitude and duration of cannabinoid receptor modulation.